V-ribbed belts are widely known as friction drive belts for transmitting power of a vehicle-mounted engine to drive accessories thereof.
When such a V-ribbed belt used in an engine accessory drive system showing significant rotational variations and load is exposed to water, such as by rainfall, it may slip on the pulley to produce abnormal noise. Such production of abnormal noise can be believed to be due to the following reason: The wet belt after being exposed to water has a smaller coefficient of dynamic friction than under normal condition. When the belt condition changes from wet to dry, its coefficient of dynamic friction increases. In this case, the change from wet to dry condition rapidly and discontinuously occurs, whereby the belt has wet portions and dry portions along its circumference. Thus, a stick-slip phenomenon occurs that the belt and the pulley alternate slips and engagements between them.
Patent Document 1 discloses a V-ribbed belt in which a compression rubber layer contains cotton short fibers and para-aramid short fibers protruding from the rib side surfaces, the protruding para-aramid short fibers are fibrillated and the cotton short fibers and the para-aramid short fibers are blended at 10 to 40 parts by weight and 5 to 10 parts by weight, respectively, into 100 parts by weight of rubber of the compression rubber layer. The document describes that, according to this V-ribbed belt, when used for a motor vehicle in an engine showing significant rotational variations, the belt can prevent slight slips upon exposure to water to avoid noise production.
Since, however, the V-ribbed belt disclosed in Patent Document 1 contains a large amount of short fibers, this gives the belt itself a high rigidity, which makes it easier for a slip to occur on the belt rather than makes it harder.    Patent Document 1: Published Japanese Patent Application No. 2001-165244